Configure the option "Grammar-checking color" (set the color; File | Options)

Here you can configure the color that TextMaker uses to paint the line under words with wrong grammar. Note that you must also set the option to use a user-defined color.

GrammarCheckColor

Registry Hive	HKEY_LOCAL_MACHINE or HKEY_CURRENT_USER
Registry Path	SOFTWARE\Policies\SoftMaker\TextMaker 2018
Value Name	GrammarCheckColor
Value Type	REG_DWORD
Default Value	8421504
Min Value	0
Max Value	16777215